Field of the Invention
This invention relates to an assistant apparatus for measuring concentration of a test substance in liquid suitably used for adhering a diffusion-limiting membrane which is used to increase a concentration measuring limit of a test substance and for simplifying the disposing operation of the diffusion-limiting membrane after the measuring operation. This invention also relates to a concentration measuring apparatus for measuring the test concentration of a substance in liquid, with the assistant apparatus removably instaled thereto. This invention further relates to a method for measuring concentration of a test substance in liquid using the assistant apparatus and the test apparatus.
It is known that a physiologically active substance has a caracteristic capable of selectively detecting a very complicated organic compound, protein or the like with high sensibility. With attention directed to this characteristic, research and development have been made on measurement of such organic compounds, proteins or the like with the use of an enzyme electrode unit having base electrodes on which a physiologically active substance (hereinafter referred to as an enzyme) is immobilized.
When measuring a test substance in a liquid with the use of the enzyme electrode unit above-mentioned, the test substance is oxidized or reduced in the presence of such enzyme. The concentration of the test substance is determined by measuring the amount of a substance produced or consumed in such oxidation or reduction. For instance, when the concentration of glucose is measured using an enzyme electrode consisting of a glucose oxidase as an emzyme and a platinum electrode and a silver electrode as base electrodes, the following reaction occurs. ##STR1## As is apparent from the reaction equation, oxygen is consumed and hydrogen peroxide is produced. So the concentration of glucose is determined by measuring the amount of produced hydrogen peroxide or consumed oxygen.
Accordingly, the upper limit of concentration which can be measured is determined dependent on the amount of a substance provoking such oxidation or reduction, for example the amount of oxygen.
In view of the foregoing, it has been proposed to increase the concentration measuring limit by limiting the penetration rate of a test substance by a diffusion-limiting membrane mounted on the surface of an enzyme-immobilized membrane.
More specifically, there has been adopted an arrangement in which the diffusion-limiting membrane is mounted on a cap to be threadably secured to the base portion of a rod-like enzyme electrode unit, and screwing the cap causes the diffusion-limiting membrane to be automatically contacted with the enzyme-immobilized membrane.
With the use of such an arrangement, the penetration rate of a test substance to be measured is limited by the diffusion-limiting membrane, thereby to achieve measurement of a considerably high concentration. That is, an output signal from the enzyme electrode is lowered by limiting the penetration rate of a test substance, but the output signal is hard to saturate notwithstanding the increase of concentration of a test substance in a liquid. Furthermore the concentration measuring limit is decided by the saturation point. As a result, the concentration measuring limit is increased up to the concentration corresponding to the saturation point. To eliminate the influence of interfering substances contained in a test solution to be measured (for example, increase in diffusion limiting effect resulting from the adhering of such interfering substances), the diffusion-limiting membrane needs to be replaceable. This is the reason for adopting the cap membrane screwing mechanism.(Japanese Patent Laid open No.Sho 63-243863)
When the diffusion-limiting membrane holding means having the arrangement above-mentioned is used, replacement of the diffusion-limiting membrane may be relatively facilitated. There are instances, however, where it becomes very difficult to mount or remove the holding means on or from the base portion of an enzyme electrode unit due to the arrangement of its mounting mechanism, or where it is not possible to achieve a uniform contact of the diffusion-limiting membrane to the enzyme-immobilized membrane due to the degree of the screwing force.
Further, when the enzyme electrode unit base portion has a small diameter, resulting a in decrease in the cap size, this causes the manual mounting/removal operation to be very difficult. This makes the problems above-mentioned more serious.
Moreover, the diffusion-limiting membrane is mounted on a cap, requiring a large space for preserving and/or transporting the same.
Considering above points, it is proposed to employ a diffusion-limiting membrane holder which is constructed with a thin plate having a through hole and a diffusion-limiting membrane adhered to one side of the thin plate to cover the through hole, and measuring the concentration of the test solution under the condition of pressure contacting the diffusion-limiting membrane to the surface of the enzyme electrode unit (refer to U.S. patent application No. 176,288, filed Mar. 31, 1988, now abandoned). Difficulties are encountered in dropping the solution including test substance (hereinafter referred to as test solution) to the through hole through the diffusion-limiting membrane is difficult and pressure contacting the diffusion-limiting membrane to the surface of the enzyme electrode unit. Also, upon completion of the measurement as above-mentioned, a relatively great amount of interfering substances are adhered to the diffusion-limiting membrane. This inevitably degrades the diffusion limiting effect of the diffusion-limiting membrane which is to limit diffusion of a test substance to be measured. Therefore, the diffusion-limiting membrane as it is, cannot assure an accurate measurement on and after the second operation. Accordingly, it is a common practice that, after a predetermined number of measurements have been made, preferably after every measurement has been made, the diffusion-liming membrane is exchanged with new one to achieve measurement without any influence of the interfering substances. As a result, a greater quantity of thin plate than would be needed for contacting the diffusion-limiting membrane must be used. Further, the disposal of used thin plates and the selection of new thin plate is needed. This complicates the series of operations to measure the concentration of the test substance.
More specifically, a series of operations is needed as follows
(1) a power switch provided with the concentration measuring apparatus is turned on,
(2) a cover provided with the concentration measuring apparatus is opened,
(3) a package is opened and a thin plate on which a diffusion-limiting membrane is adhered is picked out,
(4) test solution is dropped to the thin plate (if the test solution is blood, blood is drawn, then the blood is deposited to the thin plate),
(5) the thin plate is inserted into the concentration measuring apparatus,
(6) the diffusion-limiting membrane adhered to the inserted thin plate is pressure contacted to the surface of the electrode for measuring concentration, then the concentration of the test substance is measured,
(7) after measurement is made, the diffusion-limiting membrane is released from the surface of the electrode,
(8) the thin plate is pulled out from the concentration measuring apparatus,
(9) the cover provided with the concentration measuring apparatus is closed,
(10) the pulled out thin plate is disposed of, and
(11) the power switch is turned off.
This series of operations is needed, and particularly time consuming operations are needed, thereby complicating concentration measuring operations as a whole. Further, the above-mentioned series of operations do not include a calibrating operation based on a standard solution having an established concentration of a test substance. In practice the calibrating operation is needed, so the operations are more complicated as a whole.
The description hereinbefore which has discussed mainly the case of concentration measuring of a test substance when using an enzyme electrode unit, may be also applied to the case of concentration measuring of a test substance using another electrode unit, which may also cause the similar problems.